Successive envelopes of hydrothermal alteration typically enclose a core of disseminated ore minerals in often stockwork-forming hairline fractures and veins.
32.
The mineral substitutes into portions of beryl crystals, and is either a hydrothermal alteration product or a late stage magmatic mineral.
33.
Excellent examples of hydrothermal alteration, fluidized cataclasite injection, and other mid-and upper-crustal fault processes abound along the detachment surface.
34.
Jaspillite is also formed as exhalative chemical sediments in certain lead-zinc ore deposits, and as a hydrothermal alteration facies around submarine volcanism.
35.
The main cone is formed by dacites and block-and-ash flows that were later intruded by dacitic porphyries, associated with hydrothermal alteration.
36.
The ore is a result of hydrothermal alteration of the nepheline-syenitic rocks; it has also been weathered out and concentrated in paleoplacer deposits.
37.
At least two stages of construction are recognized, Llullaillaco I and Llullaillaco II . The first stage originated from two centres and is now extensively degraded by glaciation and hydrothermal alteration.
38.
Allophane is a weathering or hydrothermal alteration product of volcanic glass and feldspars and sometimes has a composition similar to kaolinite but generally has a molar ratio of Al : Si = 2.
39.
Generally the volcanic host rocks are difficult to correlate over distances of greater than 50 m in drill core, due to their inconsistencies in texture and thickness and the overprinting hydrothermal alteration.
40.
Apart from one shallow layer that may be associated with a hydrothermal system or past hydrothermal alteration, three large structures ( volumes of ) in the shape of dykes have been identified.
